Sunday, December 10, 2006

Latose tolerance and evolution

A surprisingly recent instance of human evolution has been detectedamong the peoples of East Africa. It is the ability to digest milk inadulthood, conferred by genetic changes that occurred as recently as3,000 years ago, a team of geneticists has found.

The finding is a striking example of a cultural practice — the raisingof dairy cattle — feeding back into the human genome. It also seems tobe one of the first instances of convergent human evolution to bedocumented at the genetic level. Convergent evolution refers to two ormore populations acquiring the same trait independently.

Throughout most of human history, the ability to digest lactose, theprincipal sugar of milk, has been switched off after weaning becausethere is no further need for the lactase enzyme that breaks the sugarapart. But when cattle were first domesticated 9,000 years ago andpeople later started to consume their milk as well as their meat,natural selection would have favored anyone with a mutation that keptthe lactase gene switched on.

Such a mutation is known to have arisen among an early cattle-raisingpeople, the Funnel Beaker culture, which flourished some 5,000 to6,000 years ago in north-central Europe. People with a persistentlyactive lactase gene have no problem digesting milk and are said to belactose tolerant.

Almost all Dutch people and 99 percent of Swedes are lactose-tolerant,but the mutation becomes progressively less common in Europeans wholive at increasing distance from the ancient Funnel Beaker region.

Geneticists wondered if the lactose tolerance mutation in Europeans,first identified in 2002, had arisen among pastoral peoples elsewhere.But it seemed to be largely absent from Africa, even though pastoralpeoples there generally have some degree of tolerance.

A research team led by Sarah Tishkoff of the University of Marylandhas now resolved much of the puzzle. After testing for lactosetolerance and genetic makeup among 43 ethnic groups of East Africa,she and her colleagues have found three new mutations, all independentof each other and of the European mutation, which keep the lactasegene permanently switched on.

The principal mutation, found among Nilo-Saharan-speaking ethnicgroups of Kenya and Tanzania, arose 2,700 to 6,800 years ago,according to genetic estimates, Dr. Tishkoff's group is to report inthe journal Nature Genetics on Monday. This fits well witharchaeological evidence suggesting that pastoral peoples from thenorth reached northern Kenya about 4,500 years ago and southern Kenyaand Tanzania 3,300 years ago.

Two other mutations were found, among the Beja people of northeasternSudan and tribes of the same language family, Afro-Asiatic, innorthern Kenya.

Genetic evidence shows that the mutations conferred an enormousselective advantage on their owners, enabling them to leave almost 10times as many descendants as people without them. The mutations havecreated "one of the strongest genetic signatures of natural selectionyet reported in humans," the researchers write.

The survival advantage was so powerful perhaps because those with themutations not only gained extra energy from lactose but also, indrought conditions, would have benefited from the water in milk.People who were lactose-intolerant could have risked losing water fromdiarrhea, Dr. Tishkoff said.

Diane Gifford-Gonzalez, an archaeologist at the University ofCalifornia, Santa Cruz, said the new findings were "very exciting"because they "showed the speed with which a genetic mutation can befavored under conditions of strong natural selection, demonstratingthe possible rate of evolutionary change in humans."

The genetic data fitted in well, she said, with archaeological andlinguistic evidence about the spread of pastoralism in Africa. Thefirst clear evidence of cattle in Africa is from a site 8,000 yearsold in northwestern Sudan. Cattle there were domesticatedindependently from two other domestications, in the Near East and theIndus valley of India.

Both Nilo-Saharan speakers in Sudan and their Cushitic-speakingneighbors in the Red Sea hills probably domesticated cattle at thesame time, since each has an independent vocabulary for cattle items,said Dr. Christopher Ehret, an expert on African languages and historyat the University of California, Los Angeles. Descendants of eachgroup moved southward and would have met again in Kenya, Dr. Ehretsaid.

Dr. Tishkoff detected lactose tolerance among both Cushitic speakersand Nilo-Saharan groups in Kenya. Cushitic is a branch ofAfro-Asiatic, the language family that includes Arabic, Hebrew andancient Egyptian.

Dr. Jonathan Pritchard, a statistical geneticist at the University ofChicago and the co-author of the new article, said that there weremany signals of natural selection in the human genome, but that it wasusually hard to know what was being selected for. In this case Dr.Tishkoff had clearly defined the driving force, he said.

The mutations Dr. Tishkoff detected are not in the lactase gene itselfbut a nearby region of the DNA that controls the activation of thegene. The finding that different ethnic groups in East Africa havedifferent mutations is one instance of their varied evolutionaryhistory and their exposure to many different selective pressures, Dr.Tishkoff said.

"There is a lot of genetic variation between groups in Africa,reflecting the different environments in which they live, from desertsto tropics, and their exposure to very different selective forces,"she said.

People in different regions of the world have evolved independentlysince dispersing from the ancestral human population in northeastAfrica 50,000 years ago, a process that has led to the emergence ofdifferent races. But much of this differentiation at the level of DNAmay have led to the same physical result.

As Dr. Tishkoff has found in the case of lactose tolerance, evolutionmay use the different mutations available to it in each population toreach the same goal when each is subjected to the same selectivepressure. "I think it's reasonable to assume this will be a moregeneral paradigm," Dr. Pritchard said.